As a technique to detect a load imposed on each of vehicle wheels of an automotive vehicle, a sensor-equipped wheel support bearing assembly has also been suggested, in which the load is detected by detecting a strain occurring in a flange outer diametric face of an outer ring of the wheel support bearing assembly that serves as a stationary ring. (See, for example, the Patent Document 1 listed below.). In addition, a wheel support bearing assembly including a strain gauge provided on an outer ring of the wheel support bearing assembly for the detection of a strain has been suggested. (See, for example, the Patent Document 2 listed below.)
Further, the sensor equipped wheel support bearing assembly has also hitherto been suggested, in which a sensor unit made up of a strain generating member and a strain sensor fitted to this strain generating member is fitted to the stationary ring of the bearing unit, which strain generating member includes a contact fixing segment at least two locations relative to the stationary ring and has a cutout defined at least one location between the neighboring contact fixing segments, with the strain sensor arranged in this cutout. (See, for example, the Patent Document 3 listed below.)
According to the sensor equipped wheel support bearing assembly disclosed in the Patent Document 3 spura, when the load acts on a rotatable ring incident to travel of the automotive vehicle, the stationary ring is deformed by the load acting through rolling elements and such deformation brings a strain in the sensor unit. The strain sensor provided in the sensor unit detects such strain in the sensor unit. If relations between strains and loads are determined beforehand by means of a series of experiments and/or simulations, the load or the like acting on the vehicle wheel can be detected from an output of the strain sensor.
[Patent Document 1] JP Laid-open Patent Publication No. 2002-098138
[Patent Document 2] JP Published International Application No. 2003-530565
[Patent Document 3] JP Laid-open Patent Publication No. 2007-057299
According to the technique disclosed in the Patent Document 2 listed above, detection is made of the strain generated as a result of deformation occurring in the flange on the stationary ring. A problem has, however, been found that since slippage occurs between the flange surface and the knuckle surface in the event that a force exceeding a stationary frictional force acts, application of repeated load results in a hysteresis appearing in an output signal.
By way of example, in the event that the load acting in a certain direction relative to the wheel support bearing assembly increases, no slippage occurs at first between the stationary ring flange surface and the knuckle surface because the stationary frictional force is larger than the load, but in the event that it exceeds a certain magnitude, the stationary frictional force overcomes, resulting in slippage. When starting from this condition the load is reduced, such a hysteresis appears in the output signal. Once this hysteresis appears, the detecting resolution will be lowered.
Also, in the case where the strain gauge is pasted to the outer ring such as disclosed in the Patent Document 2 spura, a problem has been found in assemblability. In the sensor equipped wheel support bearing assembly disclosed in the Patent Document 3, supra, the amplitude of the output signal of the sensor unit tends to increase each time the rolling elements of the wheel support bearing assembly move past in the vicinity of an area of installation of the sensor unit. In other words, the output signal of the sensor unit comes to represent a periodic waveform that is affected by the rolling elements and, therefore, the load cannot be detected with high accuracy.